U.S. patent number 9,287,469 [Application Number 12/151,089] was granted by the patent office on 2016-03-15 for encapsulation for phosphor-converted white light emitting diode.
This patent grant is currently assigned to CREE, INC.. The grantee listed for this patent is Arpan Chakraborty. Invention is credited to Arpan Chakraborty.
United States Patent |
9,287,469 |
Chakraborty |
March 15, 2016 |
Encapsulation for phosphor-converted white light emitting diode
Abstract
An improved light emitting device, especially a
phosphor-converted white light device, wherein the light extraction
efficiency and the color temperature distribution uniformity are
improved by the introduction of both nanoparticles and light
scattering particles proximate to the light source. Nanoparticles
having a high index of refraction are dispersed throughout a
wavelength conversion layer to adjust the index of refraction of
the layer for improved light extraction. Light scattering particles
may be dispersed in the wavelength conversion layer and/or in a
surrounding medium to improve the spatial correlated color
temperature uniformity.
Inventors: |
Chakraborty; Arpan (Goleta,
CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Chakraborty; Arpan |
Goleta |
CA |
US |
|
|
Assignee: |
CREE, INC. (Durham,
NC)
|
Family
ID: |
41057288 |
Appl.
No.: |
12/151,089 |
Filed: |
May 2, 2008 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090272996 A1 |
Nov 5, 2009 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
33/501 (20130101); H01L 33/56 (20130101); H01L
2933/0091 (20130101); H01L 33/60 (20130101); H01L
2924/0002 (20130101); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
H01L
33/44 (20100101); H01L 33/50 (20100101); H01L
33/60 (20100101) |
Field of
Search: |
;257/99,100,E23.119
;438/27 |
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dated Nov. 17, 2015. cited by applicant .
Office Action from U.S. Appl. No. 11/895,573, dated Oct. 7, 2015.
cited by applicant .
Response to OA from U.S. Appl. No. 11/895,573, filed Dec. 16, 2015.
cited by applicant .
Office Action from U.S. Appl. No. 13/169,866, dated Nov. 5, 2015.
cited by applicant.
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Primary Examiner: Ingham; John C
Attorney, Agent or Firm: Koppel Patrick Heybl &
Philpott
Claims
I claim:
1. A light emitting diode (LED) device, comprising: an LED chip on
a mount surface; a conversion layer on said LED chip, said
conversion layer comprising a binder material and a plurality of
phosphor particles throughout said binder material; a plurality of
nanoparticles throughout said binder material such that the
effective refractive index of a combination of said binder material
and said nanoparticles is between the refractive index of said
phosphor particles and the refractive index of a surrounding
medium; and a light scattering layer on said conversion layer and
in contact with said mount surface, said light scattering layer
comprising a plurality of light scattering particles throughout
said surrounding medium, the refractive index of said light
scattering particles being greater or less than the refractive
index of said surrounding medium, wherein light conversion only
takes place in the conversion layer.
2. The LED device of claim 1, wherein said conversion layer
comprises greater than 5% nanoparticles by volume.
3. The LED device of claim 1, wherein said conversion layer
comprises greater than 10% nanoparticles by volume.
4. The LED device of claim 1, wherein said conversion layer
comprises 20-40% nanoparticles by volume.
5. The LED device of claim 1, wherein said conversion layer
comprises approximately 20% nanoparticles by volume.
6. The LED device of claim 1, said light scattering particles
comprising particles comprising a diameter of approximately 0.1-2
mm.
7. The LED device of claim 1, wherein said conversion layer
comprises approximately 0.01-5% light scattering particles by
volume.
8. The LED device of claim 1, further comprising an encapsulant
substantially enveloping said LED chip and said conversion layer,
such that said encapsulant comprises said light scattering
layer.
9. The LED device of claim 8, wherein additional light scattering
particles are throughout said encapsulant.
10. The LED device of claim 8, wherein said light scattering
particles are throughout said binder material and said
encapsulant.
11. The LED device of claim 1, wherein said nanoparticles are
throughout said binder material such that the effective refractive
index of said combination of said binder material and said
nanoparticles is approximately 1.7.
12. The LED device of claim 1, further comprising a spacer layer
interposed between said LED chip and said conversion layer.
13. The LED device of claim 1, wherein said nanoparticles comprise
titanium (IV) dioxide (TiO.sub.2) particles.
14. The LED device of claim 1, wherein said nanoparticles comprise
diamond particles.
15. The LED device of claim 1, wherein a primary emission surface
of said LED chip is textured.
16. The LED device of claim 1, further comprising a reflective
element on said LED chip opposite said conversion layer.
17. The LED device of claim 1, wherein said LED chip is nitride
based.
18. The LED device of claim 1, wherein said nanoparticles have a
higher refractive index than said binder material.
19. A light emitting diode (LED) device, comprising: an LED chip on
a mount surface, said LED chip comprising a textured primary
emission surface; a wavelength conversion layer comprising a flat
primary emission surface on said LED chip, wherein light conversion
only takes place within said wavelength conversion layer, said
conversion layer comprising: a binder material comprising a first
refractive index; a plurality of phosphor particles throughout said
binder material; a plurality of nanoparticles throughout said
binder material such that the effective refractive index of a
combination of said binder material and said nanoparticles is
between the refractive index of said phosphor particles and the
refractive index of a surrounding medium; and an encapsulant over
said wavelength conversion material and in contact with said mount
surface, said encapsulant comprising a plurality of light
scattering particles throughout said surrounding medium, the
refractive index of said light scattering particles being greater
or less than the refractive index of said surrounding medium.
20. The LED device of claim 19, wherein said conversion layer
comprises approximately 0.01-5% light scattering particles by
volume.
21. The LED device of claim 19, wherein said encapsulant
substantially envelopes said LED chip.
22. The LED device of claim 21, wherein additional light scattering
particles are throughout said encapsulant.
23. The LED device of claim 21, wherein said light scattering
particles are throughout said binder material and said
encapsulant.
24. The LED device of claim 19, wherein said nanoparticles are
throughout said binder material such that the effective refractive
index of said combination of said binder material and said
nanoparticles is approximately 1.7.
25. The LED device of claim 19, wherein said conversion layer
comprises greater than 5% nanoparticles by volume.
26. The LED device of claim 19, wherein said conversion layer
comprises greater than 10% nanoparticles by volume.
27. The LED device of claim 19, wherein said conversion layer
comprises 20-40% nanoparticles by volume.
28. The LED device of claim 19, wherein said conversion layer
comprises approximately 20% nanoparticles by volume.
29. The LED device of claim 19, said nanoparticles comprising
titanium dioxide (TiO.sub.2) particles.
30. The LED device of claim 19, said nanoparticles comprising
diamond particles.
31. The LED device of claim 19, said light scattering particles
comprising fused silica particles.
32. The LED device of claim 19, wherein said light scattering
particles have a diameter of approximately 0.1-2 mm.
33. The LED device of claim 32, said light scattering particles
comprising titanium dioxide (TiO.sub.2) particles.
34. The LED device of claim 32, said light scattering particles
comprising diamond particles.
35. The LED device of claim 32, said light scattering particles
comprising fused silica particles.
36. The LED device of claim 19, further comprising a reflective
element on said LED chip opposite said conversion layer.
37. The LED device of claim 19, wherein said LED chip comprises
aluminum gallium indium nitride (AlGaInN) semiconductor layers.
38. The LED device of claim 19, wherein said nanoparticles have a
higher refractive index than said binder material.
39. The LED device of claim 19, further comprising a spacer layer
interposed between said textured primary emission surface and said
conversion layer.
40. A light emitting diode (LED) device, comprising: an LED chip on
a mount surface, said LED chip comprising a textured primary
emission surface; a wavelength conversion layer on said LED chip,
wherein said light conversion only takes place within said
wavelength conversion layer, said conversion layer comprising: a
binder material comprising a first refractive index; a plurality of
phosphor particles throughout said binder material, wherein said
plurality of phosphor particles comprises a mixture of phosphor
particles comprising a diameter of less than approximately 10
nanometers and phosphor particles comprising a diameter greater
than approximately 10 micrometers; and a spacer layer interposed
between said textured primary emission surface and said conversion
layer, wherein said spacer layer has an index of refraction less
than said conversion layer and said LED chip, wherein said spacer
layer is conformal to said LED chip and wherein said wavelength
conversion material is conformal to said spacer layer; and an
encapsulant over said LED chip and in contact with said mount
surface comprising a plurality of light scattering particles
throughout said encapsulant, the refractive index of said light
scattering particles being greater or less than the refractive
index of said encapsulant.
41. The LED device of claim 40, wherein said conversion layer
comprises approximately 0.01-5% light scattering particles by
volume.
42. The LED device of claim 40, wherein said encapsulant
substantially envelopes said LED chip.
43. The LED device of claim 42, wherein additional light scattering
particles are throughout said encapsulant.
44. The LED device of claim 40, wherein said light scattering
particles are throughout said binder material and said
encapsulant.
45. The LED device of claim 40, said light scattering particles
comprising titanium dioxide (TiO2) particles comprising a diameter
of approximately 0.1-2 mm.
46. The LED device of claim 40, said light scattering particles
comprising diamond particles comprising a diameter of approximately
0.1-2 mm.
47. The LED device of claim 40, said light scattering particles
comprising fused silica particles comprising a diameter of
approximately 0.1-2 mm.
48. The LED device of claim 40, further comprising a reflective
element on said LED chip opposite said conversion layer.
49. The LED device of claim 40, wherein said LED chip comprises
aluminum gallium indium nitride (AlGaInN) semiconductor layers.
50. A light emitting diode (LED) device, comprising: an LED chip on
a mount surface; a conversion layer on said LED chip, said
conversion layer comprising a binder material and a plurality of
phosphor particles throughout said binder material, wherein light
conversion only takes place within said conversion layer; a
plurality of nanoparticles throughout said binder material such
that the effective refractive index of a combination of said binder
material and said nanoparticles is increased; and an encapsulant
over said conversion layer and in contact with said mount surface,
said encapsulant comprising a plurality of light scattering
particles throughout said encapsulant, the refractive index of said
light scattering particles being greater or less than the
refractive index of said encapsulant; wherein said effective
refractive index is between the refractive index of said phosphor
particles and the refractive index of said encapsulant.
51. The LED device of claim 50, wherein said nanoparticles have a
diameter smaller than the wavelength of light emitted from said LED
chip.
52. The LED device of claim 50, wherein said nanoparticles comprise
particles comprising a diameter less than about 10 nm.
53. The LED device of claim 50, wherein said nanoparticles are
configured to downconvert at least some light emitted from said LED
chip.
54. The LED device of claim 50, wherein said nanoparticles exhibit
excitation in the blue and/or UV emission spectrum.
55. The LED device of claim 50, wherein said LED chip is configured
to emit blue light; and wherein said nanoparticles are configured
to absorb at least some of said blue light and re-emit red
light.
56. The LED device of claim 50, wherein said nanoparticles comprise
a second plurality of phosphor particles.
57. The LED device of claim 50, wherein said binder material is
thermally conductive.
58. The LED device of claim 50, wherein said conversion layer is on
a thermally conductive reflective element.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to light emitting diodes
(LEDs) and, more particularly, to LEDs having nanoparticles and
light scattering particles that improve the light extraction
efficiency and spatial color temperature uniformity of the LED
device.
2. Description of the Related Art
Solid state light emitting devices, such as inorganic or organic
light emitting diodes (LEDs), convert energy to light and are
widely used for many applications. As known to those having skill
in the art, inorganic solid state devices generally include one or
more active regions of semiconductor material interposed between
oppositely doped regions. When a bias is applied across the doped
regions, electron-hole recombination events occur that generate
light, and light is emitted from the active region in
omnidirectional paths from all surfaces of the LED. Conventional
LEDs may incorporate reflectors and/or mirror surfaces to direct
the emitted light in a desired direction.
The color or wavelength emitted by an LED is largely dependent on
the properties of the material from which it is generated that
determine the bandgap of the active region. LEDs have been built to
emit light in a range of colors in the visible spectrum including
red, yellow, green, and blue. Other LEDs emit radiation outside the
visible spectrum such as in the ultraviolet (UV) range of the
electromagnetic spectrum. It is often desirable to incorporate
phosphors into a LED to tailor the emission spectrum by converting
a portion of the light from the LED before it is emitted. For
example, in some blue LEDs a portion of the blue light is
"downconverted" to yellow light. Thus, the LED emits a combination
of blue and yellow light to generate a spectrum that appears white
to the human eye. As used herein, the term "phosphor" is used
generically to indicate any photoluminescent material.
Phosphors have been disposed in various regions within the LED
structure. For example, phosphor may be dispersed inside and/or
coated outside a dome-shaped encapsulant that covers the device.
The phosphor may be located remotely from the light emitting die as
shown in U.S. Pat. No. 7,286,926. The phosphor may also be coated
or deposited on the die itself. Several techniques are frequently
used to introduce the phosphor, including electrophoretic
deposition, stencil printing, spin or spray coating, etc. Another
technique uses a phosphor dispense process where a drop of
material, such as epoxy, silicone encapsulant, etc., that contains
phosphor therein, may be placed on the die and cured to form a
shell over the die. This is sometimes is sometimes referred to as a
"glob top" process. In another technique, the drop of material that
contains phosphor may be placed above the die, and the phosphor is
allowed to settle within the drop. This technique may be referred
to as "remote settling".
Many applications require an LED that emits white light. As used
herein, the term "white light" is used in a general sense and
includes light that different individuals or detectors may perceive
as having a slight tint toward, for example, yellow or blue. As
discussed above, some conventional LED devices combine a yellow
phosphor on a blue LED to achieve white light. Some of the blue
light emitted from the LED passes through the phosphor without
being converted, and some of the emitted blue light is
downconverted to yellow. The combinations of blue light and yellow
light that escape the light emitting device provide a white light
output.
LEDs have been manufactured that include several other functional
features, such as reflective/refractive layers, lenses and light
scattering elements, for example. Some LEDs include surfaces that
have been textured to enhance light extraction by reducing total
internal reflection at various material interfaces. Many other
functional features known in the art may be combined to build an
LED having particular characteristics.
FIG. 1 shows a cross-sectional view of known LED device 10. An LED
chip 11 is disposed on a mount surface 12. A layer of wavelength
conversion material 13 surrounds the LED chip 11. An encapsulant 14
covers the LED chip 11 and the conversion layer 13. The LED chip
has a textured emission surface 15 that helps to extract light at
the interface of the LED chip 11 and the conversion layer 13 by
countering the effects of the step-down in index of refraction at
the interface. Phosphor particles 16 are shown dispersed throughout
the conversion layer 13. Some of the light emitted from the LED
chip 11 is reflected or backscattered inside the conversion layer
13, at the interface with encapsulant 14, or within the encapsulant
14 back towards the textured surface 12. Due to the textured
surface 15 coupled with a step-up in index of refraction at the
interface, a substantial portion of the light incident on the
textured surface 15 re-enters the LED chip 11 where it may be
absorbed, decreasing the light extraction efficiency of the device
10.
SUMMARY OF THE INVENTION
In one embodiment, a light emitting diode (LED) device comprises
the following. An LED chip has a conversion layer disposed on it,
the conversion layer having a binder material and a plurality of
phosphor particles dispersed throughout the binder material. A
plurality of nanoparticles is disposed proximate to the LED chip. A
plurality of light scattering particles is disposed proximate to
the LED chip.
In another embodiment, a light emitting diode (LED) device
comprises the following. An LED chip is disposed on a mount
surface, the LED chip having a textured primary emission surface. A
wavelength conversion layer is disposed on the LED chip. The
conversion layer comprises: a binder material having a first
refractive index; a plurality of phosphor particles dispersed
throughout the binder material; and a plurality of nanoparticles
dispersed throughout the binder material. A plurality of light
scattering particles is arranged proximate to the LED chip.
One method of manufacturing a plurality of light emitting diode
(LED) devices comprises the following. A plurality of LED devices
is grown on a semiconductor wafer. A wavelength conversion layer is
applied on the wafer such that each of the LED devices is covered
by the conversion layer, the conversion layer comprising a
plurality of nanoparticles dispersed throughout the conversion
layer. A plurality of light scattering particles is arranged
proximate to the LED devices.
Another embodiment of a light emitting diode (LED) device comprises
the following. An LED chip is disposed on a mount surface, the LED
chip having a textured primary emission surface. A wavelength
conversion layer is disposed on the LED chip. The conversion layer
comprises: a binder material having a first refractive index; a
plurality of phosphor particles dispersed throughout the binder
material, wherein the phosphor particles have a diameter of less
than 10 nanometers or greater than 10 micrometers. A plurality of
light scattering particles is arranged proximate to the LED
chip.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a known LED device.
FIG. 2 is a cross-sectional view of an LED device according to an
embodiment of the present invention.
FIG. 3 is a cross-sectional view of an LED device according to an
embodiment of the present invention.
FIG. 4 is a cross-sectional view of plurality of LED devices
according to an embodiment of the present invention.
FIG. 5 is a cross-sectional view of an LED device according to an
embodiment of the present invention.
FIG. 6 is a cross-sectional view of an LED device according to an
embodiment of the present invention.
FIGS. 7A-7E each show a cross-sectional view of an LED device at
the wafer level during various stages of a fabrication process.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention provide an improved light
emitting device, especially a white light device, wherein the light
extraction efficiency and the color temperature distribution
uniformity are improved by the introduction of both nanoparticles
and light scattering particles proximate to the light source.
Nanoparticles having a high index of refraction are dispersed
throughout a wavelength conversion layer to adjust the index of
refraction of the layer for improved light extraction. Light
scattering particles may be dispersed in the wavelength conversion
layer or in a surrounding medium to improve the spatial correlated
color temperature (CCT) distribution uniformity.
It is understood that when an element such as a layer, region or
substrate is referred to as being "on" another element, it can be
directly on the other element or intervening elements may also be
present. Furthermore, relative terms such as "inner", "outer",
"upper", "above", "lower", "beneath", and "below", and similar
terms, may be used herein to describe a relationship of one layer
or region to another. It is understood that these terms are
intended to encompass different orientations of the device in
addition to the orientation depicted in the figures.
Although the ordinal terms first, second, etc., may be used herein
to describe various elements, components, regions, layers and/or
sections, these elements, components, regions, layers and/or
sections should not be limited by these terms. These terms are only
used to distinguish one element, component, region, layer or
section from another. Thus, a first element, component, region,
layer or section discussed below could be termed a second element,
component, region, layer or section without departing from the
teachings of the present invention.
It is noted that the terms "layer" and "layers" are used
interchangeably throughout the application. A person of ordinary
skill in the art will understand that a single "layer" of material
may actually comprise several individual layers of material.
Likewise, several "layers" of material may be considered
functionally as a single layer. In other words the term "layer"
does not denote an homogenous layer of material. A single "layer"
may contain various material concentrations and compositions that
are localized in sub-layers. These sub-layers may be formed in a
single formation step or in multiple steps. Unless specifically
stated otherwise, it is not intended to limit the scope of the
invention as embodied in the claims by describing an element as
comprising a "layer" or "layers" of material.
Embodiments of the invention are described herein with reference to
cross-sectional view illustrations that are schematic illustrations
of idealized embodiments of the invention. As such, variations from
the shapes of the illustrations as a result, for example, of
manufacturing techniques and/or tolerances are expected.
Embodiments of the invention should not be construed as limited to
the particular shapes of the regions or particles illustrated
herein but are to include deviations in shapes that result, for
example, from manufacturing. A region illustrated or described as
rectangular, for example, will typically have rounded or curved
features due to normal manufacturing techniques. Thus, the regions
illustrated in the figures are schematic in nature; their shapes
are not intended to illustrate the precise shape of a region or
particle and are not intended to limit the scope of the invention.
The elements are not shown in scale relative to each other but,
rather, are shown generally to convey spatial and functional
relationships.
The term "light" as used herein is not limited to electromagnetic
radiation within the visible spectrum. For convenience, "light" may
also include portions of the electromagnetic spectrum outside the
visible spectrum, such as the infrared or ultraviolet spectra, for
example.
FIG. 2 shows a cross-sectional view of an LED device 200 according
to an embodiment of the present invention. An LED chip 202 is
disposed on a mount surface. The LED 202 can emit light of
different colors and can also emit radiation outside the visible
spectrum, such as infrared or ultraviolet. The color of the emitted
light is determined by the material properties of the semiconductor
used in the chip 202. The LED chip 202 can be made from many
different material systems with one suitable material being gallium
nitride (GaN). In one embodiment the LED chip 202 emits blue light.
As discussed above, many applications require a white light output
which may be achieved by downconverting a portion of the blue light
to yellow light. When emitted, the combination of blue and yellow
light appears white.
The conversion process takes place in the conversion layer 204
which may be loaded with phosphors. The wavelength conversion layer
204 converts a portion of the emitted light to a different
wavelength, a process that is known in the art. Yttrium aluminum
garnet (YAG) is an example of a common phosphor that may be
used.
In one embodiment, the phosphor comprises many different
compositions and phosphor materials alone or in combination. A
single crystalline phosphor can comprise yttrium aluminum garnet
(YAG, with chemical formula Y.sub.3Al.sub.5O.sub.12). The YAG host
can be combined with other compounds to achieve the desired
emission wavelength. In one embodiment where the single crystalline
phosphor absorbs blue light and re-emits yellow, the single
crystalline phosphor can comprise YAG:Ce. This embodiment is
particularly applicable to LEDs that emit a white light combination
of blue and yellow light. A full range of broad yellow spectral
emission is possible using conversion particles made of phosphors
based on the (Gd,Y).sub.3(Al,Ga).sub.5O.sub.12:Ce system, which
include Y.sub.3Al.sub.5O.sub.12:Ce (YAG). Other yellow phosphors
that can be used for white emitting LED chips include:
Tb.sub.3-xRE.sub.xO.sub.12:Ce (TAG); RE=Y, Gd, La, Lu; or
Sr.sub.2-x-yBa.sub.xCa.sub.ySiO.sub.4:Eu.
Other compounds can be used with a YAG host for absorption and
re-emission of different wavelengths of light. For example, a
YAG:Nb single crystal phosphor can be provided to absorb blue light
and re-emit red light. First and second phosphors can also be
combined for higher CRI white (i.e., warm white) with the yellow
phosphors above combined with red phosphors. Various red phosphors
can be used including: Sr.sub.xCa.sub.1-xS:Eu, Y; Y=halide;
CaSiAlN.sub.3:Eu; or Sr.sub.2-yCa.sub.ySiO.sub.4:Eu.
Other phosphors can be used to create saturated color emission by
converting substantially all light to a particular color. For
example, the following phosphors can be used to generate green
saturated light: SrGa.sub.2S.sub.4:Eu;
Sr.sub.2-yBa.sub.ySiO.sub.4:Eu; or SrSi.sub.2O.sub.2N.sub.2:Eu.
The following lists some additional suitable phosphors that can be
used as conversion particles, although others can be used. Each
exhibits excitation in the blue and/or UV emission spectrum,
provides a desirable peak emission, has efficient light conversion,
and has acceptable Stokes shift: YELLOW/GREEN (Sr,Ca,Ba)
(Al,Ga).sub.2S.sub.4:Eu.sup.2+
Ba.sub.2(Mg,Zn)Si.sub.2O.sub.7:Eu.sup.2+
Gd.sub.0.46Sr.sub.0.31Al.sub.1.23O.sub.xF.sub.1.38:Eu.sup.2+.sub.0.06
(Ba.sub.1-x-ySr.sub.xCa.sub.y)SiO.sub.4:Eu
Ba.sub.2SiO.sub.4:Eu.sup.2+ RED Lu.sub.2O.sub.3:Eu.sup.3+
(Sr.sub.2-xLa.sub.x) (Ce.sub.1-xEu.sub.x)O.sub.4
Sr.sub.2Ce.sub.1-xEu.sub.xO.sub.4 Sr.sub.2-xEu.sub.xCeO.sub.4
SrTiO.sub.3:Pr.sup.3+,Ga.sup.3+ CaAlSiN.sub.3:Eu.sup.2+
Sr.sub.2Si.sub.5N.sub.8:Eu.sup.2+ Within the conversion layer 204
phosphor particles (not shown) are dispersed throughout a binder
material 206 such as silicone. The refractive index (RI) of the
phosphor particles and the RI of the binder 206 can be the same, or
they can be different. However, the difference is usually minimized
to reduce scattering.
A plurality of nanoparticles 208 is disposed proximate to the LED
chip 202. The nanoparticles 208 may be dispersed throughout the
binder material 206 of the conversion layer 204. By including
nanoparticles with a RI higher than that of the host medium--the
binder material 206, in this embodiment--the effective RI of the
host medium can be increased. This is done to more closely match
the RI of the binder material 206 (e.g., silicone with an
RI.apprxeq.1.5) with the RI of the phosphor particles
(RI.apprxeq.1.8). When these two elements are not closely
index-matched, the difference in RI results in light scattering as
the typical phosphor particles are substantially larger (.about.5.5
.mu.m) than the wavelength of light emitted from the LED chip 202
(e.g., .about.450 nm for a blue emitter). Light extraction
efficiency increases as the difference in RI between the phosphor
and the binder material 206 is reduced. However, the efficiency
only increases up to a point. If the effective RI of the binder
material 208 get too high, the light extraction efficiency will
actually decrease due to total internal reflection at the flat
interface of the conversion layer 204 and any surrounding medium
having a lower RI (e.g., silicone or air). One acceptable effective
RI for the conversion layer is approximately 1.7, providing
improved index-matching with manageable levels of total internal
reflection.
The nanoparticles 208 may comprise several different materials. One
acceptable material is titanium dioxide (TiO.sub.2). Other
acceptable materials include diamond, silicon carbide, silicon
nanoparticles, and others. The RI of both TiO.sub.2 and diamond is
approximately 2.5. The volume of nanoparticles that is needed to
adjust the effective RI of the conversion layer by a certain amount
can be easily calculated using Vegard's Law which states that the
relationship between volume and RI is linear. For example, if the
conversion layer material has a RI of 1.5 and the target effective
RI is 1.7, then the conversion layer 204 should comprise
approximately 20% TiO.sub.2 nanoparticles by volume. Other material
combinations and compositions may also be used. For example some
embodiments may have greater than 5% nanoparticles by volume. Other
embodiments may have greater than 10% nanoparticles by volume.
Still other embodiments may include 20-40% by volume nanoparticle
loading. The concentration of the nanoparticles depends on factors
such as the material being used and the desired RI adjustment.
Index-matching the binder material 206 with the phosphor particles
reduces scattering within the conversion layer 204. This reduction
in scattering adversely affects the uniformity of the color
temperature distribution in the far-field. To compensate for this
negative effect, light scattering particles are introduced into the
device.
Light scattering particles (LSPs) 210 are disposed proximate to the
LED chip 202. This technique is known in the art and only briefly
discussed herein. The LSPs 210 are distributed around the LED chip
202 so that the individual photons are redirected before they are
emitted to randomize the point where they exit the device 200. This
has the effect of evening out the color temperature distribution
such that an outside observer sees roughly the same color over a
broad range of viewing angles. Embodiments of the present invention
provide good spatial color temperature uniformity which is a range
of less than 500K at cool-white temperatures (5000-7000K) and a
range of less than 300K for warm-white temperatures
(2500-4000K).
The LSPs 210 can comprise many different materials, including:
silica gel;
silicon nanoparticles;
zinc oxide (ZnO);
yttrium oxide (Y.sub.2O.sub.3);
titanium dioxide (TiO.sub.2);
barium sulfate (BaSO.sub.4);
alumina (Al.sub.2O.sub.3);
fused silica (SiO.sub.2);
fumed silica (SiO.sub.2);
aluminum nitride;
glass beads;
diamond;
zirconium dioxide (ZrO.sub.2);
silicon carbide (SiC);
tantalum oxide (TaO.sub.5);
silicon nitride (Si.sub.3N.sub.4);
niobium oxide (Nb.sub.2O.sub.5);
boron nitride (BN); or
phosphor particles (e.g., YAG:Ce, BOSE)
Other materials not listed may also be used. Various combinations
of materials or combinations of different forms of the same
material may be used to achieve a particular scattering effect.
These LSPs 210 should have a high index of refraction relative to
the surrounding medium 212, creating a large index of refraction
differential between the materials. Because the index differential
causes refraction, it would also be possible to use an LSP material
that has a low index of refraction relative to the surrounding
medium. The LSPs 210 create localized non-uniformities in the
medium that force the light to deviate from a straight path.
When the light strikes one or more of the scattering particles 210
the index of refraction differential between the medium and the
particles 210 causes the light to refract and travel in a different
direction. A large index of refraction differential yields a more
drastic direction change for an incident photon. For this reason,
materials with a high index of refraction work well in mediums such
as silicone or epoxy. Another consideration when choosing a light
scattering material is the optical absorbance of the material.
Large particles backscatter more of the light inside the package
before it can escape the device 200, decreasing the total luminous
output of the device. Thus, preferred scattering particle materials
have a high index of refraction relative to the medium and a
particle size comparable to the wavelength of the light propagating
through the host medium. Ideally, LSPs ensure maximum forward or
sideways scattering effect for a given spectrum while minimizing
light loss due to backscattering and absorption.
The LSPs 210 can be dispersed throughout various elements so long
as they are proximate to the LED chip 202 such that substantially
all of the emitted light has a probability of interacting with the
LSPs 210. For example, in the embodiment shown in FIG. 2 the LSPs
210 are disposed throughout the binder material 206 along with the
nanoparticles 208 and the phosphor (not shown). Because the
conversion layer 204 is disposed on the LED chip 202, substantially
all of the emitted light travels through the conversion layer where
the LSPs 210 are dispersed before being emitted from the device
200. The LSPs 210 may also be disposed in other elements as
discussed in more detail below.
Various percentages of composition of the LSPs 210 can be used as
dictated by the application. Depending on the materials used, the
LSPs 210 will typically be found in concentrations ranging from
0.01% to 5% by volume. Other concentrations can be used; however,
the loss due to absorption increases with the concentration of the
scattering particles. Thus, the concentrations of the LSPs should
be chosen in order to maintain an acceptable loss figure.
In some embodiments the light scattering particles have diameters
that range from 0.1 .mu.m to 2 .mu.m. In some cases it may be
desirable to use LSPs of different sizes. For example, in one
embodiment a first plurality of LSPs may comprise titania, silica
and diamond, and a second plurality of LSPs may comprise fused
silica, titania and diamond. Many other combinations are possible
to achieve a desired color temperature distribution.
The device 200 includes an overmolded encapsulant 212 that is
disposed over the LED chip 202 and the conversion layer 204.
Encapsulants, often used to improve package light extraction
efficiency and to protect the LED and the conformal layers, are
known in the art and may be made from several materials such as
silicone, for example. The encapsulant 212 may also function as
beam-shaping element, such as a lens. This particular encapsulant
212 is hemispherical, although other shapes are possible.
The LED chip 202 has a textured light emission surface 214 included
to increase light extraction from the LED chip 202 by reducing
total internal reflection (TIR). There are many known techniques
that may be used to modify a surface of a semiconductor material. A
surface may be modified by an additive process wherein a material
is added to the surface which gives it a modified texture. A
surface can also be modified with subtractive processes wherein
material is removed from the surface to create the modified
texture. Subtractive processes, such as etching, cutting and
grinding, are known in the art and frequently used to texture a
surface. Opposite the textured light emission surface 214, the LED
chip 202 has a reflective element 216 such as a layer of reflective
material. The reflective element 216 redirects some of the light
generated in the LED chip 202 back towards the textured light
emission surface 214. The reflective element 214 may comprise a
thick silver mirror, for example.
FIG. 3 shows a cross-sectional view of another LED device 300
according to an embodiment of the present invention. The LED device
300 shares several common elements with the LED device 200. In this
particular embodiment, the LSPs 302 are dispersed throughout the
encapsulant 212. In this configuration, the LSPs scatter the light
that has passed from the conversion layer 204 into the encapsulant
212. The LSPs 302 may be dispersed uniformly throughout the
encapsulant, or they may be clustered in various regions to achieve
a particular output profile.
FIG. 4 is a cross-sectional view of a plurality of unsingulated LED
devices 400 according to an embodiment of the present invention.
The portion shown comprises three LED devices 402, 404, 406 formed
on a substrate 408. Dashed vertical lines represent cut-lines along
which the devices 402, 404, 406 will be singulated. The
semiconductor layers 410 are grown on the substrate 408. The
surface of the semiconductor layers 410 opposite the substrate 408
is a textured surface 412.
A conversion layer 414 is disposed on the textured surface 412. The
conversion layer 414 may be applied with a spin-coating process,
for example. The conversion layer 414 comprises phosphor particles
(not shown) within a binder material 416 such as silicone, for
example. A plurality of nanoparticles 418 is dispersed throughout
the binder material 416. The nanoparticles 418 can be mixed into
the binder material 416 with the phosphor and then spin-coated onto
the semiconductor layers 410. Other methods of mixing the
nanoparticles 418 with the binder material 416 and applying the
conversion layer 414 may also be used.
A flat encapsulant 420 is disposed on the conversion layer 414. The
encapsulant 420 may be made of epoxy, silicone, or other suitable
materials. In this particular embodiment LSPs 422 are dispersed
throughout the encapsulant 420. In another embodiment the LSPs 422
may be dispersed in the conversion layer 414. As discussed above,
the LSPs 422 improve the color temperature distribution uniformity
of the output profile. The emission surface 424 of the encapsulant
420 can be smooth or it can be textured to reduce TIR at the
encapsulant/air interface. The flat encapsulant 420 may be designed
to shape the outgoing light beam.
After the plurality of devices 400 is finished, the individual
devices 402, 404, 406 are singulated using known processes. The
devices 402, 404, 406 may then undergo packaging processes to yield
a finished product; however, all the elements shown in FIG. 4 may
be introduced at the wafer level.
FIG. 5 shows a cross-sectional view of an LED device 500 according
to an embodiment of the present invention. The device 500 comprises
several similar elements as the device 200 shown in FIG. 2. In this
particular embodiment, a spacer layer 502 is interposed between the
LED chip 202 and the conversion layer 204. The spacer layer 502 can
be made of many different materials such as silicone, epoxy, oil,
dielectrics, and other materials. The material should be chosen
such that the RI of the spacer layer 502 is smaller than the RI of
the LED chip 202 and the RI of the conversion layer 204.
The spacer layer 502 may increase light extraction efficiency when
the phosphor and the binder are not exactly index-matched. As
discussed above, the nanoparticles 208 are used to adjust the
effective RI of the conversion layer 204. However, the spacer layer
502 can compensate for those cases where the nanoparticles 208 do
not produce a large enough RI shift in the conversion layer 204 to
match the phosphor. For example, in some embodiments it may be
desirable to load the conversion layer 204 with a high volume of
phosphors. There would then be less space within the conversion
layer 204 for nanoparticles. The spacer layer 502 can work in
concert with the nanoparticles 208 to reduce the amount of light
that is reflected back into the LED chip 202 where it may be
absorbed. To perform this function, the spacer layer 502 is chosen
to have an RI that is less than the conversion layer 204. A portion
of the light that enters the spacer layer 502 is then incident on
the boundary between the spacer layer 502 and the conversion layer
204. At this boundary the light sees a step-up in RI and passes
into the conversion layer 204 with minimal reflection. If the light
is reflected or backscattered in the conversion layer 204, it will
see the RI step-down at the spacer layer 502 boundary and has a
finite chance of being reflected back into the conversion layer 204
because of the TIR phenomenon.
FIG. 6 illustrates a cross-sectional view of an LED device 600
according to another embodiment of the present invention. The
device 600 comprises several similar elements as the device 200
shown in FIG. 2. An LED chip 202 is disposed on a mount surface. A
wavelength conversion layer 204 covers the LED chip 202. In this
particular embodiment, scattering within the conversion layer 204
is minimized by using phosphor particles that have a relatively
large or relatively small diameter in comparison to the wavelength
of light emitted from the LED chip 202. Small phosphor particles
602 and large phosphor particles 604 are distributed throughout the
binder material 206 of the conversion layer 204. All of the
phosphor particles 602, 604 are capable of producing conversion
events.
In one embodiment, the small phosphor particles should have a
diameter of less than 10 nm; the large phosphor particles should
have a diameter greater than 10 .mu.m. It is known in the art that
particles that have diameters much larger or smaller than the
wavelength of incident light have a diminished scattering effect on
that light. Similarly as the conversion layers having
nanoparticles, the conversion layer 204 in the device 600 is
substantially scatter-free, improving the overall extraction
efficiency of the device. As discussed above, in order to ensure a
uniform color temperature distribution in the output, light
scattering particles 606 are dispersed proximate to the LED chip
202. In this embodiment, the light scattering particles 606 are
distributed throughout the encapsulant 212. In other embodiments,
the light scattering particles 212 may be dispersed in the
conversion layer 204 along with the small and large phosphor
particles 602, 604 or in both the conversion layer 204 and the
encapsulant 212.
One advantage of the embodiments discussed herein is the ability to
manufacture the improved LED devices at the wafer level. FIGS.
7a-7f show a series of cross-sectional views of two LED devices 700
according to an embodiment of the present invention during
intermediate stages of the wafer-level fabrication process.
Referring to FIG. 7A, the LED devices 700 are shown at the wafer
level. That is, the devices 700 have not been through all the steps
necessary before being separated/singulated from wafer into
individual devices. A vertical dashed line is included to show a
separation or dicing line between the LED devices 700, and,
following additional fabrication steps, the LED devices 700 can be
separated as shown in FIG. 7E. FIGS. 7A-7E show only two devices at
the wafer level; however, it is understood that many more LED
devices can be formed from a single wafer. For example, when
fabricating LED devices having a 1 millimeter (mm) square size, up
to 4500 LED devices can be fabricated on a 3 inch wafer. Typical
thicknesses for LED chips may be 50 .mu.m, 100 .mu.m, 200 .mu.m,
and as high as 600 .mu.m and beyond.
Each of the LED devices 700 comprises a semiconductor LED 202 as
described for previous embodiments. The LED devices 700 are shown
as separate devices on a substrate 701. This separation can be
achieved by having portions of the semiconductor LED layer etched
to form open areas between the LED 202. In other embodiments, the
semiconductor layer may remain continuous on the substrate 701 and
can be separated when the LED devices 700 are singulated.
A substrate may be a growth substrate or a carrier substrate. In
some embodiments, the substrate may have a diameter of about 3
inches. It is understood that substrates may come in many sizes
larger and smaller than 3 inches, all of which may be used in
various embodiments of the present invention. A growth substrate
typically is used in the fabrication of the semiconductor LED layer
and may be made of many materials such as sapphire, silicon
carbide, silicon, silicon on insulator, germanium, aluminum
nitride, and gallium nitride. A suitable growth substrate for a
Group III-Nitride based semiconductor layers is 4 H or 6 H polytype
silicon carbide, although other silicon carbide polytypes may be
used including 3 C and 15 R. Silicon carbide substrates are
available from Cree, Inc., of Durham, N.C. and methods for
producing them are set forth in the scientific literature as well
as in U.S. Pat. Nos. Re. 34,861; 4,946,547; and 5,200,022.
The substrate 701 in FIGS. 7A-7E may also be a carrier substrate.
Suitable carrier substrates may include silicon, metal alloys such
as copper alloys, single crystalline phosphors, etc. For these
embodiments, the semiconductor LED 202 is flip-wafer or flip-chip
bonded to the carrier substrate 701. A flip-wafer or flip-chip
process typically involves first growing an LED including, in
order, a n-type region, an active region, and by a p-type region on
a growth substrate and then transferring or mounting it to the
carrier substrate. The transferring/mounting step usually involves
flipping the LED device and growth substrate over and mounting it
such that the p-type region is closest to the carrier substrate.
The LED may be mounted to the carrier substrate by one or more
bonding/metal layers. The bonding/metal layers may comprise a
reflective element 216 arranged to reflect light emitted or
reflected in a direction toward the substrate 701 back toward the
top of the device. The growth substrate may then be removed using a
known grinding and/or etching process that may expose at least a
portion of the n-type region. The n-type region may then be
patterned, shaped, or textured to enhance light extraction. In
other embodiments, the growth substrate or at least portions
thereof remain. The growth substrate or the remaining portions may
then be patterned, shaped, or textured to enhance light extraction.
Example manufacturing techniques may be described in U.S.
Publication No. 20060049411 filed May 18, 2004 to Nakamura et al,
entitled Method For Fabricating Group-III Nitride Devices and
Devices Fabricated Using Method, and U.S. Publication No.
20060189098 filed Feb. 23, 2005 to Edmond, entitled Substrate
Removal Process For High Light Extraction LEDs, and U.S.
Publication No. 20060060877 filed Sep. 22, 2004 to Edmond et al,
entitled High Efficiency Group III Nitride-Silicon Carbide Light
Emitting Diode, and U.S. Publication No. 20060060874 filed Mar. 17,
2005 to Edmond et al, entitled High Efficiency Group III Nitride
LED With Lenticular Surface, all assigned to Cree, Inc.
Each of the LED devices 700 may have first and second contacts 702,
704. FIGS. 7A-7E show vertical geometry devices with the first
contact 702 being on the substrate 701 and the second contact 704
on the LED 202. The first contact 702 is shown as one layer on the
substrate 701, but when the LED devices 700 are singulated, the
first contact 702 will also be separated such that each device 700
has its own portion of the first contact 702. It is understood that
first and second contacts 702, 704 may comprise a metal such as
platinum (Pt), silver (Ag), nickel (Ni), an alloy thereof, or a
transparent conductive oxide such as indium tin oxide (ITO), zinc
oxide (ZnO), etc. As would be understood by one of skill in the art
with the benefit of this disclosure, the present invention may also
be used with LEDs having a lateral geometry wherein both contacts
are on the same side of the device.
Referring now to FIG. 7B, a contact pedestal 706 may be formed on
the second contact 704 that is utilized to make electrical contact
to the second contact 704 after various coatings are applied. The
pedestal 706 may be formed of many different electrically
conductive materials and can be formed using many different known
physical or chemical deposition processes such as electroplating,
electroless plating, or stud bumping, with a suitable contact
pedestal 706 being gold (Au) and formed by stud bumping. This
method is typically the easiest and most cost effective approach.
The pedestal 706 can be made of other conductive materials beyond
Au, such as copper (Cu), nickel (Ni), Indium (In), or combinations
thereof. For the vertical geometry type device 700 shown in FIGS.
7A-7E, only one pedestal 706 is needed. In alternative embodiments
related to lateral geometry or large-area vertical geometry
devices, additional pedestals may also be formed.
The process of forming stud bumps is generally known and only
discussed briefly herein. Stud bumps are placed on the contacts
(bond pads) through a modification of the "ball bonding" process
used in conventional wire bonding. In ball bonding, the tip of the
bond wire is melted to form a sphere. The wire bonding tool presses
this sphere against the contact, applying mechanical force, heat,
and/or ultrasonic energy to create a metallic connection. The wire
bonding tool next extends the gold wire to the connection pad on
the board, substrate, or lead frame, and makes a "stitch" bond to
that pad, and finishes by breaking off the bond wire to begin
another cycle. For stud bumping, the first ball bond is made as
described, but the wire is then broken close to the ball. The
resulting gold ball, or "stud bump" remains on the contact and
provides a permanent, reliable connection through the underlying
contact metal. The stud bumps can then be flattened (or "coined")
by mechanical pressure to provide a flatter top surface and more
uniform bump heights, while at the same time pressing any remaining
wire into the ball.
In FIG. 4C, the devices 700 are then blanketed by a conversion
layer 708 (similarly as the conversion layer 414 shown in FIG. 4)
using techniques such as electrophoretic deposition, spin or spray
coating, screen printing, and/or dispensing. Although not shown in
FIGS. 7A-7F, the conversion layer 708 may contain nanoparticles,
light scattering particles, or combinations of both, as discussed
in detail above. The conversion layer 708 may also comprise
phosphors of different size. The conversion layer 708 has a
thickness such that it may cover/bury the pedestal 706. One
advantage of depositing the conversion layer 708 at the wafer level
is the elimination of the need for precise alignment over
particular devices or features. Instead, the entire wafer is
covered, resulting in a simpler and more cost effective fabrication
process.
Referring now to FIG. 7D, the conversion layer 708 is thinned or
planarized so that the pedestals 706 are exposed through the top
surface of the conversion layer 708. Many different thinning
processes can be used including known mechanical or chemical
processes such as grinding, lapping, polishing, the use of a
squeegee, and physical or chemical etching. The thinning process
not only exposes the pedestals 706, but also allows for planarizing
and controlling the thickness of the light conversion layer 708,
which in some embodiments may control the intensity and the color
point of the emitted light by controlling the amount of phosphor
present.
Referring now to FIG. 7E, individual devices 700 may be singulated
using known methods such as dicing, scribe and breaking, or
etching. The singulating process separates each of the devices 700
with each having substantially the same thickness of conversion
layer 708. This allows for reliable and consistent fabrication of
LED devices 700 having similar emission characteristics. Following
singulation, the LED devices 700 may be mounted in a package, or to
a submount or printed circuit board (PCB) without the need for
further processing to add phosphor or scattering elements. In one
embodiment the package/submount/PCB may have conventional package
leads with the pedestals electrically connected to the leads. A
conventional encapsulant (such as the encapsulant 212 shown in FIG.
2) may then surround the LED chip and electrical connections. In
another embodiment, the LED chip may be enclosed by a hermetically
sealed cover with an inert atmosphere surrounding the LED chip at
or below atmospheric pressure.
Many additional embodiments are possible which feature additional
elements. For example, LED devices according to embodiments of the
present invention may include wavelength-specific filter layers,
custom electrical contacts, lenses, and many other elements. It is
understood that embodiments of the present invention are not
limited to those exemplary combinations disclosed herein.
Although the present invention has been described in detail with
reference to certain preferred configurations thereof, other
versions are possible. Therefore, the spirit and scope of the
invention should not be limited to the versions described
herein.
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